Such apparatuses and methods are also known under the headword "Heating Element Butt Welding". Thereby, the welding operation is performed in two phases in chronological sequence, i.e. a heating phase and a jointing phase.
Common to most of already known apparatuses for the welding of thermoplastic plastic articles, particularly plastic pipes, is that the jointing pressure is controlled during the cooling period for monitoring the welding operation. This sequence is recorded in a way-pressure-force-time-diagram. Hitherto, it was considered as ensured that a process control monitored in such a way is essential for the quality of the welding.
In numerous investigations, it has been observed that also other influences are essential for a welding of high quality and reproducibility.